1. Field of the Invention
The present invention relates to a liquid ejection head that ejects liquid such as ink and also to a recording apparatus that operates for recording on a recording medium by means of such a liquid ejection head.
2. Description of the Related Art
More and more inkjet type recording apparatus have been employed in recent years and are currently being employed for so-called large format printing to exploit the advantages of inkjet recording by ejecting liquid such as ink. Large format printing refers to printing (recording) on recording mediums having a relatively large printing area such as large posters to be used for advertisements of events and various presentations. Some recording apparatus capable of recording on an about 2 m wide recording medium have been put to practical use.
Recording mediums for large format printing have a relatively large recording area. Therefore, recording apparatus for large format printing are required to operate at high speed for printing. For this reason, liquid ejection heads utilizing electrothermal transducer elements that can accommodate high speed printing are adopted in recording apparatus for large format printing.
Now, the configuration and the operation of a liquid ejection head utilizing electrothermal transducer elements will be described below.
The liquid ejection head includes a recording element substrate having electrothermal transducer elements and a supporting member for supporting the recording element substrate. The recording element substrate is formed with a supply port for supplying ink to the inside and the supporting member is formed with a supply flow path communicating with the supply port. Additionally, the recording element substrate is formed with a plurality of ejection ports for ejecting the ink supplied through the supply flow path.
The liquid ejection head is so designed as to apply electric energy to the electrothermal transducer elements according to the recording signals received from the recording apparatus main body and rapidly raise the temperature of electrothermal transducer elements. The thermal energy of the electrothermal transducer elements is transmitted to the ink inside the recording element substrate to make the ink represent a phase change. The air bubble pressure generated as a result of the phase change of the ink is converted into ejection energy so that the ink in the recording element substrate is ejected from the ejection ports.
In a liquid ejection head utilizing electrothermal transducer elements, the electric energy applied to the electrothermal transducer elements is partly accumulated in the recording element substrate as thermal energy. The thermal energy accumulated in the recording element substrate is then transmitted to the outside of the liquid ejection head by way of the supporting member. When the liquid ejection head is driven to eject ink continuously, the quantity of the thermal energy applied from the electrothermal transducer elements to the recording element substrate is greater than the quantity of the thermal energy transmitted from the recording element substrate to the supporting member to consequently raise the temperature of the recording element substrate.
Particularly, the part of the recording element substrate that faces the opening of the supply flow path is not held in contact with the supporting member. Therefore, the thermal energy of that part of the recording element substrate is hardly transmitted to the supporting member if compared with the part of the recording element substrate that is held in contact with the supporting member. In other words, the temperature of the former part of the recording element substrate tends to be raised easily.
When the temperature of the recording element substrate rises above a certain level, the ink ejecting operation becomes unstable to by turn deteriorate the quality of the image recorded by the liquid ejection head. Additionally, the temperature of the parts of the recording element substrate located close to the ejection ports and the quantity of the ejected ink are correlated and, if the temperature in the inside of the recording element substrate varies to a large extent, the ink ejection rates of the ejection ports will also vary among them to a large extent. Then, as a result, uneven image density is caused in the recorded image to consequently deteriorate the quality of the recorded image.
As an attempt to prevent such image quality deterioration, recording apparatus including a liquid ejection head that utilizes electrothermal transducer elements are mostly so controlled as to temporarily suspend the recording operation before the temperature of the recording element substrate rises above a certain level or becomes to be dispersed to a large extent. Temperature rises and temperature variance in the recording element substrate are suppressed by suspending the recording operation of the recording apparatus to secure the time required to emit thermal energy from the recording element substrate and also the time required to flatten the temperature distribution in the recording element substrate.
However, with large format printing, the image recorded on a recording medium more often than not spreads continuously in the recording area. When the recording operation is suspended while recording a continuous image, the tint of the ink ejected on the recording medium can vary in the continuous image to deteriorate the quality of the recorded image. For this reason, liquid ejection heads for large format printing are required to have a structure that does not give rise to temperature rises and temperature variance in the recording element substrate if a recording operation is conducted continuously for a relatively long period of time.
Japanese Patent Application Laid-Open Publication No. 2009-90572 discloses an exemplar liquid ejection head that can effectively suppress temperature rises and temperature variance in its recording element substrate. The liquid ejection head disclosed in Japanese Patent Application Laid-Open Publication No. 2009-90572 includes a plurality of beams held in contact with the recording element substrate. Since the heat in the part of the recording element substrate that faces the opening of the supply flow path is mostly released to the outside by way of the beams, the temperature rise of that part is suppressed. Additionally, since thermal energy is transmitted from the inside of the recording element substrate to the beams at a plurality of spots, the temperature variance in the recording element substrate is suppressed.
However, Japanese Patent Application Laid-Open Publication No. 2009-90572 discloses only a liquid ejection head having a single supply flow path in its supporting member. The inventors of the present invention have found that a novel problem arises when a plurality of supply flow paths are formed in a liquid ejection head disclosed in Japanese Patent Application Laid-Open Publication No. 2009-90572.
This novel problem will be described below by referring to FIGS. 10A through 10D.
FIG. 10A is a schematic plan view of a liquid ejection head of the type under consideration that has three supply flow paths in the supporting member. FIG. 10B is a schematic cross-sectional view of the liquid ejection head taken along line 10B-10B in FIG. 10A. FIG. 10C is a schematic plan view of the supporting member 3 of the liquid ejection head from which its recording element substrate has been removed.
As seen from FIGS. 10A through 10C, the liquid ejection head includes three recording element substrates 2a, 2b and 2c having respective ejection ports 1 and a supporting member 3 for supporting the recording element substrates 2a, 2b and 2c. The supporting member 3 has three supply flow paths 4 and the openings of the three supply flow paths 4 are aligned in the first direction X running along the surface of the supporting member 3.
The recording element substrates 2a, 2b and 2c are rigidly secured at positions where they cover the respective openings of the supply flow paths 4. Each of the recording element substrate 2a, 2b and 2c is formed with two supply ports 5 such that a single supply flow path 4 communicates with two supply ports 5.
The liquid ejection head has a pair of beams 6 extending along the first direction X in each of the supply flow paths 4. The pairs of beams 6 are held respectively in contact with the recording element substrates 2a, 2b and 2c through the openings of the supply flow paths 4. The paired beams 6 arranged in each of the supply flow paths 4 are separated from each other by gap D in the second direction Y intersecting the first direction X.
The inventors of the present invention computationally determined the temperature distributions in each of the recording element substrates 2a, 2b and 2c along the second direction Y. FIG. 10D is a graph illustrating the computationally determined temperature distributions.
In the graph illustrated in FIG. 10D, the horizontal axis represents temperatures and the vertical axis represents positions in the second direction Y on each of the recording element substrates 2 (positions on the temperature measurement lines M illustrated in FIG. 10A). The wide solid line in the graph indicates the temperature distribution in the recording element substrate 2b located between the other two recording element substrates as viewed in the first direction and the narrow solid line and the dotted line in the graph respectively indicate the temperature distributions in the recording element substrates 2a and 2c located at the opposite ends as viewed in the first direction X.
As seen from FIG. 10D, the temperatures in the recording element substrate 2b located between the other two recording element substrates are higher than the corresponding temperatures in the other recording element substrates 2a and 2c located at the opposite ends. This is because thermal energy is mainly transmitted from the recording element substrates 2a and 2c to the partition walls 7 separating the supply flow paths 4 and also to the walls located outside the supply flow paths 4 as viewed in the first direction X but from the recording element substrate 2b only to the partition walls 7 separating the supply flow paths 4.
The openings of the three supply flow paths 4 are respectively covered by separate recording element substrates 2a, 2b and 2c in the liquid ejection head illustrated in FIGS. 10A through 10C. If the three supply flow paths 4 are covered by a single recording element substrate, the temperatures in the center section thereof are supposed to be higher than the corresponding temperatures in the opposite end sections so that temperature distributions graph similar to the one represented in FIG. 10D may be obtained.
As illustrated in FIG. 10D, since temperature differences exist between the recording element substrate 2b and the other recording element substrates 2a and 2c, the ink ejection rate varies between the recording element substrate 2b and the other recording element substrates 2a and 2c, and therefore, uneven image density is caused in the recorded image to consequently deteriorate the quality of the recorded image.
Particularly, of recording apparatus of the type under consideration, the ejection ports 1 (FIG. 10B) arranged in an intermediate portion of each of the recording element substrates 2 as viewed in the second direction Y (in the positional range between about −3 mm and about −22 mm in the graph of FIG. 10D) are mostly used to eject ink. Therefore, if the temperatures in such an intermediate portion represent variance among the recording element substrates, uneven image density can easily be caused in the recorded image.